components: Add BUILD.gn file to metrics so we can build it with GN.

Debug:

$ gn gen out/Debug_gn --args=is_debug=true
$ ninja -C out/Debug_gn metrics

Release:

$ gn gen out/Release_gn --args=is_debug=false
$ ninja -C out/Release_gn metrics

BUG=None
TEST=see above.
R=brettw@chromium.org
TBR=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/255413004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266675 0039d316-1c4b-4281-b951-d872f2087c98

BUILD.gn

@@ -20,6 +20,7 @@ group("root") {
     "//apps/common/api:apps_api",
     #"//chrome",
     "//components/language_usage_metrics",
+    "//components/metrics",
     "//components/navigation_metrics",
     "//components/onc",
     "//components/os_crypt",

components/metrics/BUILD.gn

+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+source_set("metrics") {
+  sources = [
+    "metrics_hashes.cc",
+    "metrics_hashes.h",
+  ]
+
+  deps = [
+    "//base",
+  ]
+}